Today, I hacked a multi-function audio player (commonly known as a "boom box") to add an external audio input. This hack isn't really done yet (I still have to solder everything up, drill a hole, add the 1/8" jack, etc.), but I have pictures of what I've done.
Essentially, I started out by poking around the live circuit with my oscilloscope. I didn't get very far with that, so I decided to look up the datasheet for the identical DIP chips that looked like they were the amp chips. After reading the bits I needed, I saw that the chips were indeed amplifiers (with one for each channel). After tracing some circuit paths, I managed to find a good point to input the signal. I connected some wires to those points, found a point for ground, and connected it to an 1/8" jack in my iPhone.
I was able to play Derezzed REALLY loud before it started getting distorted.
Sorry there's no video, but the music player I was using also happens to be my video camera, phone, calendar, and video player (among other things). Also, my normal video camera wasn't available. Click "Continue Reading" to see the pictures.